This invention relates to video display systems. More particularly, this invention relates to a video display system for use with a personal computer for displaying text characters or graphic symbols on a CRT display in selectable screen formats, where the display fields for each selected format is the same size.
The techniques of displaying alphanumeric characters or graphic symbols on a CRT screen is well known in the art. Raster scan video display units have horizontal and vertical scan or sync frequencies for controlling the position of the electron beam(s), which is, in turn, modulated to create the image on the CRT screen. The images are displayed in a display field comprising some determined number of horizontal scan lines. Each horizontal scan line in the display field is divided into a number of pixel locations, or dots. The display field will consist of an array of Y by X dots, where Y is the number of dots on each horizontal scan line across the display field and X is the number of scan lines up and down the field. For example, a common display field would be 640.times.200 dots.
Control of the number of horizontal scan lines in the display field, i.e., the height of the display field is under control, for the most part, of the vertical scan frequencies. The height adjustment control of the CRT video display also affects the height of any image displayed on the screen.
Within this array of dots forming the display field, a typical prior-art video display system operating in the text or alphanumeric display mode subdivides the display field array into a determined number of alphanumeric character cells. A character cell could, for example, be an array of 8.times.8 dots, where each character is actually produced from a 7.times.7 dot matrix centered inside a character cell, thus leaving a 1 dot space between each character in the displayed field. For a display field of 640.times.200 dots and a character cell of 8.times.8 dots, it is possible to display 25 lines of text of 80 characters per line. A display field of 25 lines of 80 characters created from a 640.times.200 dot array represent what is hereinafter referred to as a screen format.
Different screen formats can be specified to obtain different results. For example, a color graphics display created on a color CRT monitor from 8.times.8 character cells in a 640.times.200 dot display field may create a color display that is pleasing to the eye with good color quality and resolution. However, the 8.times.8 character cell is not the most desirable display size for alphanumeric characters because the characters are not as well formed as where the character cell comprises more dots. It is known that a character cell formed from a 9.times.14 dot array offers a far superior optical display of text characters because of the size and sharpness of each formed character and because each character can be more completely formed.
For the most part, however, prior-art video display systems come with only one screen format capability. For example, the IBM personal computer provides a black and white monitor for its alphanumeric display with a 720.times.350 dots display field, and a separate color monitor for its color graphics with a 620.times.200 dots display field. This single screen format capability in a single monitor results from the need to precisely maintain many separate frequency signals to the display control circuitry in order to create a displayed image, i.e., the horizontal sync frequency, the vertical sync frequency, the dot clock for timing the output of the video signals for each dot displayed, etc. The frequency of these signals bear close relationships in order to display in a display field a particular screen format. Additionally, a black and white monitor (actually a green and black display, P39 phospor) provides better contrast for the alpha mode.
There have been attempts in the prior art to provide selectable screen formats in a single video display system. For example, in the case of a 640.times.200 dot display field, it is possible to change the number of lines in the display field by dividing the number of scan lines in half to obtain a display field that is 320.times.200 dots, where each dot in this field is 2 dots thick. This format can be obtained without changing the horizontal or vertical scan frequencies.
Another way to obtain a different screen format without having to change the scanning frequencies is to double the number of horizontal scan lines by interleaving horizontal scan lines. That is, after each vertical retrace, the position of all of the horizontal scan lines are indexed one-half of the normal scan lines separation, and for these lines displaying new information. The next vertical retrace causes the lines to return their previous positions. In this manner, twice as many lines can be obtained in the display field without changing the horizontal or vertical scan frequencies. However, this approach results, in most cases, in an unacceptable flicker of the display because of the slower refresh of each horizontal scan line.
These prior-art video display systems which have attempted to provide selectable different screen formats in a single monitor, operate on the frequencies and video control timing signals to obtain binary relationships therebetween. For example, in the case of changing from a 640.times.200 dot display field (80 characters per line of text) to a 320.times.200 dot field (40 characters per line of text), the number of horizontal scan lines is divided by 2. Similarly, in increasing the number of lines of the 640.times.200 dot display field (graphics) to a 640.times.400 dot display field (graphics) requires that the number of lines be multiplied by 2.
A binary relationship, however, does not exist between, for example, a 640.times.200 dot display field graphics or alpha) and a 720.times.350 dot display field (alpha only). A 720.times.350 dot display field using a 9.times.14 dot character cell will display 25 lines of 80 characters in a manner similar to the 8.times.8 dot character cell in the 640.times.200 dot display field discussed above, but with a significantly different field width and height if the timing frequencies, i.e., the horizontal and vertical scan frequencies, were to remain the same as was the case in the prior-art video display systems.
Accordingly, it would be advantageous to provide a video display system having the capability of automatically selecting and displaying multiple screen formats in the same display field size on the same or a separate video monitor when the number of horizontal lines and dots per line are not related by binary multiples, and to do so without the need of any external adjustments at the time of selecting.